Purification of metals



May 8A, 1945. P. P. ALEXANDER 2,375,199

PURIFICATION 0F METALS Filed April4 26, 1944 ATTORNEYS Patented May i945-v UNITED STATES PATENT oFrlcE Peter P. Alexander, to Metal HydridesMarblehead, Mass., assignor Incorporated, Beverly, Mass..

a corporation of Massachusetts Application April 26, 1944, Serial No.532,735

, 19 Claims. (Cl. 'i5-63) This invention relates to the purification ofmetals contaminated with metal impurities having a lower boiling point,especially oxidizable metal impurities, and has for its object moreparticularly the purification of metals contaminated with one or more ofthe alkali metals.

The alkaline-earth metals, calcium, strontium and barium, are usuallymade by subjecting their fused chlorides to electrolysis. Since thesalts of the alkali metals, sodium and potassium, are usually present inthese chlorides, the resulting alkaline-earth metal is usuallycontaminated with a small amount of alkali metal, as well as of thechloride of the alkaline-earth metal, itself. To purify thealkaline-earth metal, it has been proposed to subject the metal todistillation in a closed retort that has been evacuated to removeobjectionable air. The distilled metal is permitted to condense in acooler part of the retort, usually in the form of a more or less spongydeposit. After cooling the retort, air is admitted, the retort is openedand the condense metal is removed. The resulting product is so reactivethat it frequently oxidizes rapidly on the surface and produces heat insuiiicient amount to ignite the whole mass, with the result that firesare quite' common. 'I'his objectionable result is all the more apt tooccur because of the presence of the more highly reactive sodium andpotassium. These alkali metals are also distilled and condense in acooler part of the retort. The small deposits of sodium and potassiumare so reactive that they almost invariably catch re when the retort isopened and they come in contact -with the oxygen of air.

Magnesium is produced pyrometallurgically and electrolytically. It iscommon in both methods to encounter the alkali metals, particularlysodium and potassium because they are foundwith the magnesium in nature.With sea water or magnesium-bearing ores as the source of the magnesium,there is a marked tendency for sodium and potassium to be present inelectrolytic magnesium in amounts that are objectionable for somepurposes. When magnesium-bearing ores are subjected to reductionpyrometallurgically, for example, in a retort that hasbeen evacuated toremove objectionable air, and the reduced magnesium is distilled andcondensed, sodium and potassium tend to follow the magnesium and areusually condensed in a cooler zone. Fires are not uncommon when theretort, or its associated condenser, is opened and the sodium andpotassium rst contact the oxygen of the air.

Special precautions are taken to condense mag- 1 nesium separate fromthe sodium and potassiumand to remove the sodium and potassium withoutstarting a re. A special condenser for the sodium and potassiumisusually provided at the coolest end of the retort, removed from the zonein which the magnesium is condensed; but the condensed sodium andpotassium may nevertheless catch nre on exposure tothe air.

As a result of my. investigations I Ahave discovered that thesedisadvantages may 'be overcome for the most part. The alkali metals,sodium and potassium, may be segregated from the higherboilingpointmetal to be puried and converted into Vcompounds that arenon-inflammable on exposure to the oxidizing influence of air. The metalto be puried may be separately collected and in some instances be freedof other impurities.

In accordance with the invention, the metal to be purified is heated inthe absence of air to a temperature sufciently high to distill thealkali metal impurity. 'Ihe resulting alkalimetal vapor impurityis thenselectively oxidized. This may be accomplished by bringing the alkalimetal vapor impurity into reactive contact with a refractory metaloxide, such as titanium oxide, zirconium oxide and chromium oxide, andcollecting the oxidized alkaliA metal impurity as a solid separate fromthe metal to be puriiied. The metal to be puried is itselfadvantageously distilledin some cases and condensed separate from theoxidized alkali metal impurity.

These and other features of the invention may be better understood byreferring to the accompanying drawing, taken in conjunction with the4following description, in which:

Fig. 1 is a diagrammatic representation, being a side elevation insection, of an apparatus with which the method of the invention may bepracticed; and

Fig. 21s a horizontal section on the line 2 2 of Fig. 1.

The apparatus shown comprises an outer retort it supported Within aheating furnace il, the retort being held in position by means of two ormore supports l2 resting on top of the heating furnace? The retortpreferably |consists of heatresistant steel. It is provided with aremovable cover I3 having attached thereto a vertical pipe it andlateral valved branches I5 and i6, connectable with a, source of vacuum,and air, respectively. A removable inner retort 20, having an open top,rests within the outer retort. A removable tray 2l rests over the opentop of the retort, being supported thereon by means of three or morelugs 22 extending over the wall of the inner retort. The heating furnaceil consists essentially of a rectangular chamber 23 having a refractorybottom 24, side and end walls 25, and a top 26 having an opening of a,size adapted to receive the outer retort. An expanding opening 21 isprovided at or near a lower |corner of one of the side walls of thechamber, for the introduction of heating gases into the chamber. Asimilar opening 28 is preferably provided at or near an Opposite corner,as a, spare or auxiliary means for around the retort 2l, which is inturn placed vide asealed joint.

introducing heating, or cooling, gases into the chamber. When not inuse, the opening 2l is closed. A nue opening 29 extends through theVsame wall as the opening 21, preferably at a higher level, soithatheating gases* passed into the chamber through the opening 21 tend topass before leaving the chamber through the ilue opening 29.

The apparatus may be used as follows: A charge of calcium, for example,contaminated with sodium, potassium, and calcium chloride, is placed inthe bottom of the inner retort 20. The inner retort is then placed inthe outer retort Ill. A layer of titanium oxide 2li is placed in thetray across the open top oi l The removable cover il is placed l andlocked thereon to pro- The lateral branch Ii is then connected to asource of vacuum.v With the valve oi this lateral branch open, and thevalve of the other lateral branch` I6 closed, the outer and innerretorts are placed under vacuum to remove objectionable air.

Heating gases are passed through opening 21 into the heating chamber,where they circulate the retort and then ilue opening 29 to the outsidelower portion of the outer portion of the the inner retort. on the outerretort pass through the atmosphere., As the retort I0 is heated, thelower inner retort is also heated. As the temperature rises, a point isreached where the sodium and potassium present in the calcium aredistilled.

'to the upper and cooler portion of the retort As the temperature at thebottom of the inner retort continues to rise, a point is reached wherethe calcium is tends to condense and settle out against the wall of theinner retort immediately below the tray I6. Some calcium vapor may ofcourse rise sunlciently to come in contact with titanium dioxide in thetray and react with it in a .manner similar to sodium or potassiumvapor. The bulk of the distilled calcium may, densed and collected, asshown. sisting essentially of calcium chloride, .perhaps with a smallamount of sodium chloride or potassium chloride, or both, remains as aresidue in the bottom oi the inner retort.

The introduction of heating gases to the heating chamber is stopped andthe retort is permitted to cool. The cooling operation may beaccelerated by passing cooling air through the heating chamber, forexample, through the opening 28, or by its contents from admit air tothe interior of the retorts, after which the cover I3 is removed. Theinner retort and tray are vpulled out of the outer retort; the

distilled and its resulting vapor are advantageously removed by washingthe inner retort with water.

It will be clear to those skilled in this art that the invention lendsitself to numerous modiilcations. Other metals of higher boiling point,such as barium, strontium, magnesium, etc., may be similarly treated toremove metal impurities of lower boiling point, such as the alkalimetals, so- 'dium and potassium, to convert them into harmlesscompounds. Instead. of titanium oxide other suitable refractory metallicoxide, such as zirconium oxide and chromium oxide,A may be employed,either separately or admixed. While l silicon or ferro-silicon anddistilled in an taken oi the relative boiling points 410 elevated to apoint where the specific example relates to the purifying of a primarymetal, it will be clear` that the invention also lends itself which themetal to be purified is reduced from its compounds and the alkali metalimpurity is selectively oxidized. This modification may be employed, forexample, in conjunction with the above mentioned method of producingmagnesium pyrometallurgically. According to a wellknown practice,magnesium oxide is reduced with externally heated retort placed undervacuum to remove objectionable air. The distilled magnesium is condensedin a cooler part of the retort or in a condenser specially provided forthe purpose. The alkali metal impurities, sodium and potassium, arelikewise reduced and distilled. In accordance with the presentinvention, they are converted into harmless compounds by selectivelyoxidizing them in the manner described.

In the practice of the invention, advantage is of the metals. Theboiling points of the alkali metals, sodium and potassium, aresubstantially below those of the metals to be puriiled. The temperatureof the metal to be puried is, therefore, gradually the metal impurity ofis distilled. By heating its is caused to yield a graduated temperaturerange. Since the alkali metals have the lower boiling points, theirvapors tend selectively to rise highest in the retort. Since the metalsto be puried have the higher boiling points, their vapors do not tend torise as high in the retort. They soon meet a temperature environmentconducive to condensation. In other words, the` metals to be puried maybe made to condense in an intermediate section of the retort wherethetemperature is high enough to keep the alkali metal impurities invapor form. The alkali metal vapors, on the other hand, tend to rise toa cooler portion of the retort, where they were heretofore condensed,but where they are now converted into harmless compounds in accordancewith the invention.

This application is a continuation-impart of my co-pending applicationSerial No. 478,095, led March 5, 1943'.

I claim:

1. In the method of purifying metals of higher boiling pointcontaminated with an alkali metal impurity of lower boiling point, theimprovement which comprises heating the metal to be puriiled in theabsence of air to a temperature sufiiciently high to distill the alkalimetal impurity but not the metal to be puried, selectively oxidizing theresulting alkali metal impurity vapor lower boiling point bottom, theretort ,in a zone removed from the main body oi' metal to be puriiled,and collecting the oxidized alkali metal impurity as a solid separatefrom the metal to be puriiled.

2. Method according to claim l, in which the to a smelting operation inY to be purified is distilled.

selectively oxidized by bringing it into reactive p contact withtitanium oxide in a zone removed from the main body of metal to bepurified. A

4. Method according to claim 1, in Which the resultingvapor of thealkali meta1 impurity is selectively oxidized by bringing it intoreactive contact with zirconium oxide in a zone removed from the mainbody of meta1 to be purified.

5. Method according to daim 1, in which the resulting vapor of thealkali metal impurity is selectively oxidized by bringing it intoreactive contact with chromium oxide in a zone removed from the mainbody of metal to be purified.

6. Method according to claim 1, in which the metal to be purified isalso` distilled, and condensing the resulting vapor of the metal to bepurified as a solid separate from the oxidized alkali meta1 impurity ina zone intermediate that in which the alkali metal impurity is oxidizedand that from which the main body of metal 7. In the method of purifyingmetals of higher boiling point contaminated withA an alkali metalimpurity of lower boiling point, the improvement which comprises heatingthe, metal to be puriiiedl in the absence of air to a temperaturesu'ciently high to distill the alkalimetal impurity but not the metal tobe puried, said alkali metal impurity being at least one of the group:sodium and potassium, selectively oxidizing the resulting alkali metalimpurity vapor in a zone removed from the main body of metal to bepuried, and collecting the'oxidized alkali metal impurity as a solidseparate from the metal to be purified.

8. Method according to claim 7, in which the resulting vapor of thealkali metal impurity is selectively oxidized by bringing it intoreactive contact with a refractory metal oxide in a zone removed fromthe main body of metal to be purihed..

9. Method according to claim 7, in which the metal to be purified isalso distilled, and condensing the resulting vapor of the meta1 to bepurified as a solid separate from the oxidized alkali metal impurity ina zone intermediate that in which the alkali metal impurity is oxidizedand that from which the main body of metal to be puried is distilled.

10. In the method of purifying magnesium contaminated with an alkalimetal`impurity, the improvement which comprises heating the magnesium inthe absence of air to a temperature sufiiciently high to distill thealkali metal impurity but not the magnesium, selectively oxidizing theresulting alkali metal impurity vapor in a zone removed from the mainbody of magnesium to be puriiied, and collecting the 'oxidized alkalimeta1 impurity as a solid separate from the main body of magnesium to bepurified.

. 11. Method according to claim l0, in which the resulting vapor of thealkali metal impurity is selectively oxidized by bringing it intoreactive contact with a refractory metal oxide in a zone removed fromthe main body of magnesium to be purlfled.

12. Method according to claim 10, in which the resulting vapor of thealkali metal impurity is selectively oxidized by bringing it intoreactive contact with titanium oxide in a zone removed' from the mainbody of magnesium to be purified. 13. Method according to claim 10, in`which the resulting vapor of the alkali metal impurity isV lselectivelyoxidized by bringing it into reactive contact with zirconium oxide rin azone removed from the main body of magnesium to be purified.

14.v Method according to claim 10, in which the resulting vapor of thealkali meta1 impurity is selectively oxidized by bringing it intoreactive contact with chromium oxide in a zone removed from the mainbody of magnesium tobe puriiied.

15. Method according to claim 10, in which the magnesium to be purifiedis a1so.distilled, and condensing the resulting magnesium vapor as asolid separate from the oxidized alkali metal impurity in a zoneintermediate that in which the alkali metal impurity is oxidized andthat from which the main body of magnesium is distilled.

16. In the method of purifying magnesium contaminated with an alkalimetal impurity, the improvement which comprises heating the magnesium inthe absence of air to a temperature sufiiciently high to distill thealkali (metal impurity but not the magnesium, said alkali metal impuritybeing at least one of the group: sodium and potassium, selectivelyoxidizing the resulting 'alkali meta1 vapor impurity in a zone removedfrom the main body of magnesium to be purified,

and collecting the oxidized alkali metal impurity as a solid separatefrom the magnesium.-

\ 1'7. In the method of purifying magnesium contaminated with an alkalimeta1 impurity, the improvement which comprises heating the magnesium inthe absence of air to a temperature sufliciently high todistill thealkali metal impurity but not the magnesium, selectively oxidizing theresulting alkali metal impurity vapor by bringing it into reactivecontact with a refractory metal oxide in a zone removed from the mainbody of magnesium to be purified, collecting theoxidized alkali metalimpurity as a solid separate from the main body of magnesium to bepurified, distilling the magnesium, and condensing the resultingmagnesium vapor as a solid separate from the oxidized metal impurity ina zone intermediate that in which the alkali metal impurity is oxidizedand that from which the main body of magnesium is distilled.

18. In the method of purifying metals of relatively low boiling pointscontaminated with readily oxidizable metal impurities of lower boilingpoints, the improvement which comprises heating the meta1 to bepuriiied'in the absence of air to a temperature sufficiently high todistill the metal impurity but not the metal to be puriiiedpselectivelyoxidizing the resulting metal impurity vapor by bringing it intoreactive .contact with a reducible refractory meta1 oxide in a zoneremoved from the mainbody of meta1 to be puriiied, and collecting theoxidized metal impurity as a solid separate from the metal to bepurified.

19. Method according to the preceding claim, in which the metal to bepuriiied is also distilled, and condensing the resulting vapor of themetal to be puried as a solid separate from the oxidized metal impurityin a zone intermediate that in which the metal impurity is oxidized andthat from which the main body of metal to be purified l is distilled.

PE'I'ER P. ALEXANDER.

